Over a period of decades vacuum processes have become increasingly important in the manufacture of many products. As an example, the manufacture of integrated circuits is largely a process of depositing and selectively removing thin layers of pure materials, and many of the process steps in that manufacture are necessarily performed in a vacuum environment. Materials such as tungsten silicide are deposited on wafer slices by low pressure chemical vapor deposition (LPCVD) in which the tungsten and the silicon are derived from gases bearing metal and silicon in a process performed under partial vacuum. Aluminum alloys are deposited by evaporation and condensation and also by plasma sputtering processes for forming interconnect circuitry on integrated circuits. These latter two deposition processes are are known in the art as physical vapor deposition (PVD) processes, and are performed under relatively higher vacuum (lower pressure) conditions than LPCVD processes. There are numerous support processes such as etch cleaning of wafer surfaces, pre-heating, and annealing that are also preferrably performed in a vacuum environment in the manufacture of integrated circuits. Plasma etching processes used for removing selected portions of deposited films to form patterns for circuitry are also often performed in a vacuum.
Similar vacuum processes to those used in integrated circuit manufacture are also used in the manufacture of magnetic memory disks, magneto-optical memory disks, and optical memory disks such as the increasingly popular digital compact disks used in the music recording industry. Integrated circuits and memory disks are two imporant products among many in which processes performed under vacuum environment are extensively used. Moreover, in both integrated circuit manufacture and memory disk manufacture, the substrates used form a general class of thin, flat blanks, often round, and most usually of a size falling within an approximate diameter of 25 cm or less.
To perform a vacuum process on a workpiece, it is necessary to place the workpiece in a leak-free enclosure and to remove the air from the enclosure by the use of one or more vacuum pumps. Equipment for performing the desired process on a workpiece, such as a heated crucible for evaporating aluminum, may be placed in the same enclosure and activated after sufficient air is removed to allow the process to take place without contamination. In some cases particular process gases might be introduced after air removal, such as in chemical vapor deposition processes and plasma-sputtering processes.
Early machines for performing vacuum processes were based on a single vacuum enclosure, also often called a chamber. The chamber would typically have a single door with a seal such as an O-ring, vacuum pumps for exhausting the chamber would be connected to the chamber by pipes with sealed joints and valves designed to operate in vacuum systems without leaking, and there would be fixturing for holding workpieces in relation to processing equipment also in the enclosure. As an example, there might be an electron beam device for melting and evaporating a charge of aluminum, and fixturing in the chamber would be arranged so that workpieces placed on the fixturing would have surfaces to be coated with aluminum facing an evaporation crucible of the electron beam device. Utilities like electric power and cooling water for the processing equipment would pass across the chamber wall (the vacuum boundary) by means of especially designed feedthrough devices to prevent leakage into the vacuum enclosure.
A typical processing cycle for a single chamber machine is as follows:
1. With the access door open, one or more workpieces to be processed are placed on the fixturing in the enclosure.
2. The access door is closed and latched.
3. Valves are opened in a planned sequence to connect vacuum pumps to the enclosure volume to remove the air in the enclosure and the vacuum level is monitored with one or more vacuum gauges until the level of vacuum is attained that is necessary for the process.
4. Process equipment in the chamber is activated and the process is monitored until complete, then process equipment is de-activated.
5. A vent valve is opened to allow air back into the enclosure until the internal pressure equals one atmosphere, after which the door is opened and the processed workpieces are removed.
6. New unprocessed workpieces are placed on the fixturing and the cycle begins again.
Single chamber machines such as described above for processing a plurality of workpieces in a single processing cycle are called "batch" systems, because a bath of workpieces is processed with each machine cycle. The fixturing for holding the workpieces is usually designed to maximize the number of workpieces that can be processed in a batch cycle, and the fixturing often is manipulated to move the workpieces relative to the process equipment during processing for, among other reasons, increasing the uniformity of processing. Mechanical motion for such manipulation passes through a chamber wall by means of especially designed mechanical feedthrough devices.
One of the first improvements developed for such machines was automatic sequencing of the steps in the machine cycle. Programmable sequencers were provided that would accept signals from monitoring equipment, provide signals to open and close valves, start and stop fixture manipulation, activate processing equipment, and so forth, so that manual intervention would seldom be necessary from loading of unprocessed workpieces to unloading of finished work.
The motivation for improvements in vacuum processing equipment are often similar to those encountered for improvements in other kinds of equipment. For example, among these motivations is the desire for a higher quality process resulting in a higher quality product from the workpieces processed. Also, efficiency is important, so that one has the ability to do more processing in less time, reducing the processing cost of workpieces. Also many times a new product requires processing in some manner that can't be done or can't be done well in existing equipment.
Typically, however, even if there are several motivations for improvement, there is usually a primary factor. Generally the factor has some relationship to a particular problem or limitation that has been encountered in the past. In that regard, two major limitations have become apparent with both manually cycled and automatically cycled batch machines for vacuum processing. First, in the overall machine cycle the functions of exhausting the vacuum processing chambers, venting the enclosure after processing, loading unprocessed workpieces and unloading finished workpieces are all serial in the cycle chronology. Actual processing of the workpieces is accomplished for only a fraction of the total cycle time. Second, there are serious quality problems resulting from the fact that the processing equipment and enclosure are exposed to atmosphere every cycle. Among these problems is the fact that the process has to be re-initiated for each new batch, and repeatability of process for batch to batch may vary widely. Also, most vacuum processes result in a residue of some kind being deposited on fixtures and internal walls, and such residue incorporates air molecules and water at atmospheric pressure which are released very slowly under vacuum, a phenomenon known as "outgassing". Each time a processing chamber is exposed to atmosphere, the subsequent pumpdown proceeds a little more slowly, so there is a gradual increase, cycle to cycle, in the total cycle time. There may also be a gradual deterioration in the quality of work produced due to contamination effects of outgassing. The workpieces also trap water vapor and air and contribute to outgassing during pumpdown and processing.
An improvement known as "air-locking" was developed to combat the difficulties listed above. Air-locking is a procedure very well known in the art of vacuum processing for introducing workpieces into a vacuum chamber and removing workpieces from a vacuum chamber without substantially disturbing the vacuum environment in the chamber.
An air-locked system has at least one vacuum chamber known as an air-lock attached to the main processing chamber and connected by a sealable opening. The sealable opening between the chambers is operable from outside the chambers. Another sealable opening similar to the first communicates from outside the chambers into the air-lock. The air-lock additionally has a valved vacuum pumping system of at least one vacuum pump which is independent of the pumping system for the processing chamber, and transport equipment for moving workpieces into and out of the process chamber through the sealable opening between the two. The transport equipment is also operable from outside the chambers like the sealable opening between the chambers. A valve known as a "vent valve" or "back-to-air" valve is also necessary, communicating from outside into the air-lock.
A typical system cycle for air-locked system described above proceeds as follows:
1. Air-Lock Venting - While procesing proceeds on workpieces in the process chamber, and with the sealable opening between the chambers closed and sealed, the vent valve is opened, admitting air into the air-lock until the pressure equalizes with outside air pressure.
2. Load-Unload - The sealable opening from outside into the air-lock is unsealed and opened. Processed workpieces in the air-lock are removed and unprocessed workpieces are placed into the air-lock and positioned on the internal transport.
3. Air-Lock Pumpdown - The sealable opening to the outside of the air-lock is closed, the vent valve is closed, and valving to the air-lock pumping system is opened to exhaust air from the air-lock. Pumping proceeds until a vacuum level is attained which experience has shown to be suitable for "crossover."
4. Crossover - There are two necessary conditions for "crossover". The vacuum level in the air-lock must be such that contamination of the process chamber will not take place when the sealable opening between the two chambers is opened, and the process on the batch of workpieces in the process chamber must be complete. The process equipment is deactivated, the sealable opening between the chambers is opened (from outside the chambers), the internal transport is activated so that the batch of finished workpieces in the process chamber moves into the air-lock and the batch of unprocessed workpieces in the air-lock moves into the process chamber. The sealable opening between the two chambers is closed and sealed, and the process is activated in the process chamber for the new batch.
After cross-over and reactivation of the process, the air-lock can be vented again starting a new cycle. For each cycle completed, a batch of finished workpieces is removed from the machine.
Air locking can be performed with a variety of geometries. For instance, if two air-lock chambers are used, one on each side of a processing chamber, each with a sealable opening to the process chamber and another to the outside, a system is created in which workpieces may be introduced from one side through one of the air-lock chambers, moved through the process chamber, and removed from the other side through the second air-lock chamber. Such a system is called an in-line air-locked system. Additionally, a process chamber may be extended including internal transport devices so that workpieces may be moved between sequential process stations in the process chamber, and a plurality of processes may be performed on workpieces. The arrangement also allows continuous processing. In such a queuing system the processes being performed on the workpieces are performed simultaneously with an air lock cycle, (ie. the various processes are chronologically parallel) all on separate batches of workpieces, so each cycle still produces a finished batch, now with several processes performed; and the process chamber remains under vacuum environment at all times in operation.
A further innovation substitutes a series of individual process chambers with sealable openings between each of the chambers for the processing systems having in-line process stations. With this arrangement processes that are incompatible in a single chamber system can be performed, still with air-locks introducing workpieces at one point and removing them at another.
The advantages of air locking are several. For example, the number of workpieces processed per unit time can be increased because pumping and material handling functions can be done while processing continues. Serial processing can be accomplished, even with processes that are not compatible.
In the evolution of automated, air-locked vacuum processing systems, the specific design details of systems developed are determined by the workpieces to be processed and the nature of the processes to be performed. A system for coating architectural glass sheets which coatings for optical filtering is considerably different than a system for applying magnetic coatings to aluminum disks for computer magnetic memory storage media. The differences are in such details as the size of the chambers, the kinds of materials used for coating, the fixturing for the workpieces, and the nature of internal transport devices and carriers, among others. There is such commonality, however, in the principles of air locking and vacuum practice.
One of the motivations for innovation in design of vacuum processing equipment mentioned above was an inability to adequately process new products. There have been several product developments in the past ten years that have fallen into that category. The ever decreasing dimensions of integrated circuit components to improve device density have been on such driving force. The development of thin-film hard disks has been another. Developments of magneto-optical memory storage technologies promises to continue the trend. As part of these developments, it was found that adequate processing of many such products required, among other things, better base vacuum ability than was possible with available equipment. Higher purity process gases, higher material purity, new processing capabilities, more repeatable process control, and more flexibility in process type and sequence were other things found to be necessary to adequately process some new products and new developments of older products.
One development intended to help meet the need of demanding new product requirements was one-by-one workpiece processing. Machines were developed to move single substrates in a queuing fashion through air-locks and processing stations rather than moving carriers and fixturing with multiple substrates.
One-by-one processing provided several needed advantages. Among them were:
1. Smaller System Space Requirements - Many of the new product developments requiring new system abilities also required external conditions such as "clean rooms" that provided controlled and filtered air to remove such contaminants as dust particles. Rooms to provide the special external environment are expensive, and smaller system size helps to hold down costs. The smaller systems are also generally more accessible for servicing.
2. Elimination of Carriers and Fixturing - One-by-one design generally made obsolete the large carriers and fixturing that were used in air-locked batch systems for moving large numbers of workpieces through the system. Such carriers and fixturing, exposed to atmosphere after each cycle through a system, are a vehicle for injecting contaminants through air locks and into processing chambers. Their elimination allowed higher purity processes by virtue of lower chamber base vacuum levels.
3. More Uniform and Repeatable Process - Even with fixturing designed to manipulate workpieces relative to processing equipment as an aid to uniform processing, not every workpiece would have the same exposure and treatment as every other in a bath process. One-by-one processing ensured that each substrate could be oriented to the processing equipment in exactly the geometry and for exactly the time as each other substrate.
The development of one-by-one processing machines did not make obsolete the older automated batch and in-line batch machines, which continue to be useful for processing that was not so demanding. A good example of a large in-line, automated air-locked system is that described in U.S. Pat. No. 4,313,815. Machines of that kind are extensively used for coating large architectural glass blanks, automotive parts, and other consumer products, among other things. A good example of a one-by-one processing system is that described by U.S. Pat. No. 4,311,427, "Wafer Transfer System" issued Jan. 19, 1982 to G. L. Coad, R. H. Shaw and M. A. Hutchinson, which is a system that was developed for coating primarily aluminum alloy films on wafer slices in integrated circuit production. This latter system is a load-locked system with a common vacuum processing chamber, and does not allow isolated performance of otherwise incompatible processes.
An example of a system designed to operate one-by-one and to also provide isolated process stations is that described in U.S. Pat. No. 4,500,407, "Disk Or Wafer Handling And Coating System" issued Feb. 19, 1985 to Donald R. Boys and Walter E. Graves. The present invention comprises a system of that general kind, i.e. one-by-one material handling and processing, air-locked, and with isolatable process stations.
The evolution of vacuum processing systems in general has been a history of increased complexity and cost. The developments of systems with air locks, multiple process stations and isolatable process stations has required the addition of chambers; provision of multiple sets of process equipment; a proliferation of vacuum pumps, manifolding and valves; and complicated material handling equipment. This general trend has affected both air-locked batch systems and one-by-one processing systems. In the period from about 1970 to 1975 batch processing systems sold in a range of from about $50,000 to about $150,000. One-by-one multiple processing systems today generally sell from about $500,000 to $1,500,000, an increase by an order of magnitude. Large in-line batch systems are even higher priced, with installations costing $5 million and more.
Some of the cost increase is simply inflation, but most is due to the increase in complexity and the proliferation of required equipment. This same proliferation has other negative effects, one of which is a considerable deterioration in reliability. With any mechanical or electrical device there is a finite (though often unknown) probability of failure. Addition of mechanical and electrical devices in a system design can only have the effect of decreasing the reliability of the overall system. The result of evolution in design from single chamber manually operated systems to multi-chamber automated systems with multiple processes and multiple pumping systems has resulted in a situation where such systems have a very poor reputation for reliability, and failures in operation are commonplace.
Earlier, in describing the evolution of vacuum processing systems, it was stated that internal sealable openings between chambers through which workpieces must pass from chamber to chamber, must be operated from outside the vacuum chambers. Also, internal transfer systems for moving workpieces from place to place inside such a system must be operated and driven from outside the vacuum chambers. One reason for this is that conventional motive devices such as electrical motors, air cylinders, hydraulic linear and rotary activators, and so forth, are not operable within or compatible with the vacuum environments inside such a system. The usual accomodation is to place the motive device outside the vacuum chambers and to pass motion and power across the chamber wall into the vacuum environment by use of a mechanical motion feedthrough. If the needed motion is rotary, there is a broad selection of rotary motion feedthroughs available to designers to pass the rotary motion and power across the vacuum boundary without leakage, such as the devices designed and sold by the Ferrafluidics Corporation. If the needed motive force is linear, such as provided by the shaft of an air cylinder, sealing is most usually accomplished at the vacuum boundary by welded metal bellows attached at one end of the air cylinder shaft and at the other to the vacuum boundary wall, usually with static seal O-rings, and usually inside the vacuum chamber. Whether rotary motion or linear motion, the sealing devices and methods are complicated, expensive, and prone to failure.
Wherever mechanical feedthroughs have to be used in a vacuum processing system new potentials for vacuum leaks are created. The same is true in the use of multiple vacuum pumps and manifolds. Every additional pump, every additional valve and every additional joint in vacuum manifolding is another place a leak may appear and cause a contamination and maintenance problem. Vacuum leaks are often very difficult to detect and correct, so the fewer sealable joints and feedthrough devices that have to be used in a system the more likely the system is to be reliable in use. The system described by U.S. Pat. No. 4,500,407, for example has at least eleven mechanical motive feedthrough devices and many more potential leak sites that would be counted in a single chamber batch system of several years ago.
In systems handling workpieces in a one-by-one queuing fashion through air-locks and multiple process chambers, mechanical devices are needed to pick up each workpiece and move it from one position to the next in queue. Each such movement represents an opportunity to lose control of a workpiece, and great care must be taken in design to insure the reliability of material handling devices. A system such as described in U.S. Pat. No. 4,500,407 may have twenty or more transfers of control for each workpiece between the load and the unload end of the queue.
There are many kinds of vacuum processes performed on products like semiconductor IC wafers and thin-film computer memory disks. These include processes for depositing films and processes for removing films as well as support processes such as pre-heating and post-heating of workpieces. The first multiple process machines developed for one-by-one processing were fairly limited in process flexibility. The system of U.S. Pat. No. 4,311,427, for instance, performs all processes in a single chamber with a common pumping system. The main process is sputtering of aluminum/silicon alloy for interconnect layers. Support processes in the same chamber are ion-etch bombardment to surface clean wafers prior to coating with aluminum and pre-heating of the wafer as an aid in controlling the crystal grain structure of the deposited aluminum film for various reasons. Fueled primarily by product development it has become desireable to isolate stations so that processes requiring differing vacuum levels, process gases, and other functional differences might be performed sequentially on workpieces. Many of the desireable processes are plasma processes such as sputtering and ion-etching; and in at least some variations of such processes it is desireable that the workpiece be electrically biased. The biasing in some cases may be radio-frequency (RF) alternating potential, and in other cases direct current (DC). To do such biasing has long been a source of equipment functional design difficulty in multiple process systems. The difficulty arises from the fact that it is necessary to transport a workpiece by some mechanical device and it is also necessary to isolate the biasing of a workpiece to the particular station where biased process is to occur. Some systems insert a workpiece into an isolatable process chamber and maintain the workpiece during processing on the member used to accomplish the insertion. This requires electrical isolation of the support/insertion member so that bias potential does not travel back into the transport equipment outside the particular process chamber, and an electrical contact to be made to the workpiece or the support for the workpiece within the chamber. The contact must be broken each time a workpiece exits and re-established each time a workpiece enters the process station. Such contact devices are inefficient and unreliable. It has generally been found that it is better from a processing viewpoint to place the workpiece on a dedicated biasable pedestal in the process chamber, but to do so requires a "pick-and-place" material handling operation by which a workpiece may be placed on such a pedestal by a placement device and the placement device moved away from the pedestal before process is initiated. After processing, the device must retrieve the workpiece and exit the process chamber. A unique and reliable way to "pick-and-place", allowing biased processing on a fixed pedestal has long been needed.
With the development of one-by-one systems with isolatable processing stations there often remain design limitations on the range of process types that might be performed. It might be desired, for instance, to perform a reactive etch process in one station to remove oxide layers on a metal surface before a deposition process in another station. The deposition process might be a physical deposition, such as sputtering, or a chemical process such as low pressure CVD. This is but a single example of a very large range of processes and process sequences that might be desireable. If a system layout and design is such that the connection and placement of the pump and gas supplies is impossible or very difficult, then the usefulness of such a system is limited. In certain processes the process gases may be toxic or corrosive or both, and it is necessary to remotely place pumps and provide exhaust for "burn boxes" r other means of handling such effluent materials. A system design must take all this into account.
Systems for one-by-one isolated processing usually have a common chamber or main chamber, sometimes called a transport chamber, through which workpieces are moved from one process station to another. In such systems, much of the workpiece transport equipment operates within this main chamber enclosure. The fact that such equipment is in a vacuum enclosure implies inaccessibility, and in present day systems cleaning and other service and maintenance for the workpiece transport parts is tedious, difficult and time consuming.
In a one-by-one sequential processing system the cycle time, i.e. the time between introduction of one workpiece to the system and introduction of the next workpiece, during which time a workpiece is also removed, having experienced all the sequential processes, is necessarily the time required for the longest duration single process of the several processes being performed. With the use of single chamber load-locks the pumping time for the load-lock cannot be longer than this cycle time, and in fact is less by the time required to vent the load-lock, remove a processes workpiece, and insert a new workpiece to be processed. In many cases the time is too short to ensure an adequate pumpdown due to outgassing characteristics of surfaces in the load-lock chamber and, in particular, the surfaces of the new workpiece inserted each cycle. As an example, in a one-by-one sequential processing system for depositing aluminum/silicon material for interconnect circuitry on integrated circuits the cycle time with one deposition station is typically about sixty seconds. The material handling functions and venting of the load-lock require typically requires about thirty seconds; so the actual pumping time on the load-lock is no more than thirty seconds. Even with a very small volume load-lock chamber, for example having just enough volume to enclose the wafer workpiece and mounting mechanism, this is not long enough to achieve a vacuum level close to the usual operating level of the main chamber or processing chambers. It is typical for a main transport chamber in such a system to operate in a 10.sup.-6 Torr pressure range, and for such a lock chamber to be opened into the main transport chamber when the pressure in the lock chamber is in the 10.sup.-4 Torr range. The pressure in the lock in this situation is 100 times the pressure in the main transport chamber. The newly introduced workpieces become a vehicle for introducing contamination into the machine. An arrangement allowing one-by-one isolateable processing, but also divorcing the air-lock operation from the system process cycle would be desireable, to allow time for contaminants to be pumped away before processing.
Accordingly it is an object of the present invention to provide a system with one-by-one handling capability and isolateable processing station at a greatly decreased level of mechanical complexity than has heretofore been available.
It is another object of the invention to provide a system that can be produced at a reduced cost.
It is a further object of the invention to provide a system the will exhibit increased reliability as a result of decreased complexity.
It is another object of the invention to provide a system with a plurality of isolateable processing stations without increasing the number of dynamic mechanical feedthrough devices associated with workpiece transport.
It is also an object of the invention to reduce the potential vacuum leak sites such as those associated with dynamic mechanical feedthrough devices.
It is a further object of the invention to reduce to a minimum the number of changes in control required to move workpieces through air-locking and sequentially through a plurality of processing stations.
It is another object of the invention to provide a capability to place a workpiece onto a dedicated pedestal in a processing station without supporting the workpiece during processing on the member used for insertion; and at the same time, not requiring actuation of a device or devices for gripping and/or releasing the workpiece. This particular capability allows dedicated processing pedestals to be used with static feedthroughs for electrical biasing not requiring making or breaking electrical contact with each process cycle, at the same time retaining mechanical simplicity.
It is an additional object of the invention to provide a one-by-one isolateable processing system maximizing process flexibility by making it possible to use all the known types of vacuum pumps, and additionally to remove the pumps from the system if needed.
It is a further object to provide a system in which all of the processing stations can be serviced off-line, and additionally the main transport chamber can be easily and routinely accessed for service and maintenance.
It is also an object of the invention in an alternative embodiment to provide an ability to do one-by-one isolateable processing of workpieces and at the same time to do a much longer pumping cycle on incoming workpieces than is possible with a single workpiece load-lock.